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Semiconductor quantum cascade lasers (QCLs) that emit mid-infrared light in the wavelength range 4-9 μm are unipolar and the laser emission is due to intersubband transitions in a repeated stack of multiple quantum wells. The thermal management of these devices is a challenge. The overheating of the active region (referred to as “core” throughout this paper) in these lasers decreases the optical power and ultimately results in laser failure. In this paper, a detailed finite element-based numerical modeling of the thermal behavior of these devices as well as the measurements performed to validate the models are presented. The studies include the effect of submount material, mounting schemes such as epi-side down or epi-side up mounting, and, finally, the effect of core geometry on the thermal impedance. Various core designs, such as the split core, are analyzed. Various experiments were conducted to correlate the results with the numerical modeling by measuring the thermal impedance between the laser diode's core and the bottom of the substrate as well as the temperature change within a pulse of a distributed feedback (DFB) QCL which emits in a single longitudinal mode of narrow linewidth. The temperature of the active core of a DFB QCL can be determined by measuring the lasing frequency, which changes with the temperature of the active core. By measuring the lasing frequency as a function of time within a pump current pulse, the temperature change in the active core region can be determined and thereby thermal conductance of the laser structure. In the conclusion, various recommendations for efficient thermal performance of these QCLs are provided.